1. Field of the Invention
The invention relates to a time-multiplex digital telecommunications system designed to provide simultaneous data transfer between respective ones of a multiplicity of pairs of subscribers across dedicated temporary communications links. Specifically, the invention is directed to an optional feature of such a telecommunications system concerning multi-address service wherein an authorized subscriber can transfer a message simultaneously to a selected group of subscribers connected to a central switching system of the telecommunications system.
2. Description of the Prior Art
Fully electronic computer-controlled switching systems for handling digital telecommunications services are well-known in the art. Such switching systems are flexibly designed for use in different applications, such as world-wide telex and gentex networks and may be used either as public or private data networks or as integrated networks. A telecommunications system of this type is described in U.S. Pat. No. 4,340,775. The central switching system of this telecommunications system is composed of modular hardware and software components. The hardware essentially comprises a central processing system and a communications hardware module. The central processing system performs the main control functions, especially for establishing through-connections between pairs of data terminals or subscribers, and for terminating such connections. The communications hardware module connects pairs of subscriber lines or trunk lines and controls time-multiplex operations during the transfer of messages between through-connected subscriber lines.
The communications hardware comprises a communications controller operating under program control of the central processor, terminator group controllers forming a logic interface between the communications controller and line terminators which provide the peripheral communication links to respective subscriber lines and trunk lines. The line terminators receive data at various speeds and in different codes and present formatted data to the communications controller and transmit output data to the associated subscribers or data terminals. The central processor system and other parts of the communications hardware are duplicated in the switching system for enhancement of reliability.
These hardware modules are supported by software modules, such as an operating system and a maintenance system. A control system module is the central and control-oriented part of the operating system and accomplishes central control functions. A switching system module provides all functions for switching-oriented operations and controls all phases of a connection beginning with an initial call request and ending with final clear-down and disconnect.
Such computer-controlled switching systems are designed to be easily adjustable to different kinds of applications for different uses. The desired flexibility requires a high level of switching capabilities for handling different kinds of high speed data traffic that is achieved by the variety of features and optional facilities, such as a manual switching position used for handling calls which require operator intervention. Another one of those optional features is a central distortion system, a test module comprising a distortion sender and a distortion receiver. When requested by a subscriber terminal, the distortion sender transmits test texts with different degrees of distortion to enable transmission facilities and teleprinters to be tested and adjusted. The distortion receiver analyzes on request by a subscriber terminal any texts transmitted from the subscriber to the central exchange and notifies the subscriber terminal of the degree of distortion measured.
In data telecommunications systems, in addition to establishing connections between pairs of subscribers, there is also a demand for the possibility of transferring data between more than two subscribers. This is then referred to as a multi-address connection. The term "multi-address connection" is intended to denote that a plurality of subscribers are each capable of receiving messages transmitted from an authorized subscriber.
In known switching systems, so-called multi-address circuits are provided for this purpose in the switching system which circuits receive the data transmitted by the calling subscriber. These circuits establish connections to called subscribers and connect the calling subscriber. The messages transmitted by the calling subscribers station then reach all other subscribers participating in the multi-address connection.
In U.S. Pat. No. 4,203,001 there is described an apparatus for establishing multi-address and conference call connections. The disclosed apparatus cooperates with a telecommunications system of the same type as described hereinbefore. Such a system does no longer employ separate multi-address circuits. Instead, this switching system comprises subscriber line terminators each associated with a subscriber station, and a connection memory arranged in the communications controller for storing control data. Storage cells constitute the connection memory, each of which storage cells is associated with a respective line terminator and contains control data which are required for the transmission of messages between interconnected subscribers. The control data are generated by the central processing system and are entered in a respective storage cell for the duration of a connection. A storage cell contains among other things a number of that particular line terminator which is currently cross-connected with the line terminator to which the storage cell is allocated. This enables the communications controller to operate independently from the central processing unit during the cross-connected phase of a connection.
The multi-address apparatus known from the U.S. Pat. No. 4,203,001 comprises, in addition to the line terminators interfacing incoming and outgoing lines, a series of additional line terminators, so-called multi-address terminators. Correspondingly, additional storage cells are provided in the connection memory which are associated with these multi-address terminators. The number of multi-address terminators and, hence, also the number of additional storage cells in the connection memory, depends upon how many subscribers in total are entitled to initiate multi-address operation in the switching system, on the one hand, and the maximum number of subscribers permitted to participate in a multi-address connection, on the other hand. To this end, the multi-address terminators are combined into groups corresponding in number to the subscribers entitled to initiate a multi-address connection. The first multi-address terminator of each group is connected with all of the following multi-address terminators within the group, such that a message character transmitted by the first multi-address terminator simultaneously reaches all additional multi-address terminators of that particular group. By virtue of an address of a normal subscriber line terminator entered into the storage cells associated with the multi-address terminators during the establishment of a connection, the same message character is always transmitted to different subscribers during the cyclic interrogation of the multi-address terminators.
Assuming that a reasonable percentage of subscribers should be authorized to participate in such a multi-address service, it is apparent that this optional feature will impose quite some outlay in hardware since each participating subscriber is then directly connected to a subscriber line terminator and, in addition, temporarily associated with one multi-address terminator. This approach also has some drawbacks from the systems point of view. The switching system is capable of servicing only a limited number of line terminators, and each line terminator card which is used for multi-address service limits the possible number of subscriber line terminators and, in consequence, the number of subscriber stations which can be connected to the switching system.